This invention relates to an encoder for use in efficiently encoding a moving image into a sequence of encoded signals.
It is to be noted here that a moving image is divisible into a sequence of scenes which are placed within frames, respectively, and each of which consists of a series of picture elements. Such picture elements are given to an encoder in the form of picture element signals which are grouped into a sequence of picture frames corresponding to the scenes.
Heretofore, predictive encoding is known wherein a prediction error signal is produced by using correlation within each frame (intraframe correlation) or correlation between two successive ones of the picture frames (interframe correlation). However, the predictive encoding encounters a lot of difficulties due to a drastical variation of the picture element signals derived from the moving image. Such a drastical variation appears not only between the frames but also within a single frame locally. For example, the frames are classified into dynamically and stationarily changing frames. In addition, a single frame might often include coexistence of a stationary part, a dynamic part, a part having a strong correlation between two adjacent ones of the picture elements arranged along a vertical and/or a horizontal direction, and another part of a weak correlation between the picture elements. Under the circumstances, it is difficult to achieve a high efficiency by the use of the conventional predictive encoding.
Orthogonal transform encoding is also known wherein the picture element signals are transformed into orthogonal transform coefficients. The orthogonal transform coefficients may be recognized as frequency representations of the picture element signals between a high order or high frequency component and a low order or low frequency component. Such orthogonal transform serves to reduce energy of the high frequency component and can effectively encode the picture element signals so as to compress redundancy. With this orthogonal transform encoding, the moving image can not be encoded with a high efficiency because of such a drastical variation.
Alternatively, consideration has been directed to a method which carries out predictive encoding and orthogonal transform of a prediction error signal obtained by the predictive encoding and which will be called hybrid encoding or a hybrid method. However, various kinds of parameters must be selected and controlled so as to adaptively encode the moving image which is produced as the picture element signals accompanying the drastical variation. No disclosure has been made about a relationship among such parameters and about controlling them at all. In addition, troublesome control might be required, even if such parameters would be considered. Moreover, transmission must be carried out about a species and a change of the parameters used in encoding the moving image. This might result in an increase of an amount of information transmitted from such an encoder to a decoder.
In an article contributed by Wen-Hsiung Chen et al to "IEEE Transactions on Communications" (Vol. COM-25, No. 11, November 1977, pages 1285-1292) and entitled "Adaptive Coding of Images," a Cosine tranform adaptive coding system is disclosed wherein a sequence of digitized image signals is classified into groups by monitoring ac energy of sub-blocks. Quantization and coding might be carried out in consideration of the classification. However, no suggestion is made about selecting a sequence of transmission codes in relation to the classification.